The present invention relates to a battery pack composed of a plurality of linearly interlocked batteries.
A battery pack composed of linearly interlocked secondary batteries is chiefly used in a motor vehicle, such as a hybrid car. For the battery pack of this structure, it is important to interlock the secondary batteries securely in a low resistance state. Large connection resistance not only reduces an output from the battery pack, but also gives rise to heat generation through Joule heating; moreover, losses due to resistance make the power utilization less efficient. JP-A-10-106533 discloses a structure using a connector for a battery pack composed of linearly interlocked secondary batteries. FIG. 1 and FIG. 2 respectively show the battery pack and the connector of this publication. This battery pack uses a metal cap as a connector 190. As shown in FIG. 2, the metal cap is formed by pressing a metal plate into a shape such that a cylindrical portion 194 is coupled to the outer periphery of a flat portion 193. As shown in the cross section of FIG. 1, two secondary batteries 110 are interlocked linearly with the metal cap by jointing the flat portion 193 to a sealing plate 112 of one secondary battery 110 through spot welding, and by jointing the cylindrical portion 194 to the circumferential surface of a cylindrical exterior case 111 of the other secondary battery 110 through spot welding.
The battery pack shown in FIG. 1 needs to joint the metal cap to the secondary batteries 110 through spot welding in two steps. This is because the flat portion 193 is jointed to the sealing plate 112 of one secondary battery 110 through spot welding in one step, and the cylindrical portion 194 is jointed to the exterior case 111 of the other secondary battery 110 through spot welding in the following step. For this reason, this battery pack has a drawback in that the connecting process of the metal cap cannot be any simpler. Further, this battery pack has another drawback in that the metal cap of this structure has a distance from the spot welding positions on the flat portion 193 to those on the cylindrical portion 194, and electric resistance between the spot-welded portions on the two secondary batteries 110 is increased because of the distance.
With the aim of eliminating these drawbacks, the Applicant of the present application. developed a connector 390 in the shape as shown in FIG. 3 (Japanese Patent Application No. 2000-273647). The connector 390 is provided with welding convex portions 392 protruding from both surfaces for welding purposes. As shown in the cross section of FIG. 4, the welding convex portions 392 are connected respectively to the opposing battery end faces of linearly aligned secondary batteries 310 by welding. The connector 390 is welded to the battery end faces by flowing a welding current through the secondary batteries 310 while the connector is pinched between the secondary batteries 310. The battery pack of this structure allows the connector 390 to be welded to the opposing battery end faces of the secondary batteries 310 in a single step. This battery pack, however, has a drawback in that it is difficult to interlock adjacent secondary batteries 310 through the connector 390 securely in a robust structure. In other words, the drawback of the battery pack adopting the connector 390 to enable the interlock is that the bending strength at the connection portions is poor.
The present invention was developed with the aim of eliminating these drawbacks. An important object of the invention is therefore to provide a battery pack in which batteries can be interlocked securely in a reliable manner by welding in a simple and easy way while being electrically connected in a low resistance state.